This invention relates generally to transport refrigeration systems for refrigerating perishable cargo in the cargo space of a trailer or other mobile refrigerated container and, more particularly, to shaft horsepower output augmentation of an onboard engine for powering a refrigerant compressor of a transport refrigeration unit.
Refrigerated trucks and trailers are commonly used to transport perishable cargo, such as, for example, produce, meat, poultry, fish, dairy products, cut flowers, and other fresh or frozen perishable products stowed in a temperature-controlled space, commonly referred to as the cargo box, within the truck or trailer. In the case of refrigerated trucks, a transport refrigeration system is mounted to the truck, typically behind the truck or on the roof of the truck for maintaining a controlled temperature environment within the cargo box of the truck. In the case of refrigerated trailers, which are typically pulled behind a tractor cab, a transport refrigeration system is mounted to the trailer, typically to the front wall of the trailer for maintaining a controlled temperature environment within the cargo box of the trailer.
Conventionally, transport refrigeration systems used in connection with refrigerated trucks and refrigerated trailers include a transport refrigeration unit having a refrigerant compressor, a condenser with one or more associated condenser fans, an expansion device, and an evaporator with one or more associated evaporator fans, which are connected via appropriate refrigerant lines in a closed refrigerant flow circuit. Air or an air/gas mixture is drawn from the interior volume of the cargo box by the evaporator fan(s) associated with the evaporator, passed through the airside of the evaporator in heat exchange relationship with refrigerant whereby the refrigerant absorbs heat from the air, thereby cooling the air. The cooled air is then supplied back to the cargo box.
On commercially available transport refrigeration systems used in connection with refrigerated trucks and refrigerated trailers, the compressor, and typically other components of the transport refrigeration unit, must be powered during transit by a prime mover. In the case of refrigerated trailers, the prime mover typically comprises a diesel engine carried on and considered part of the transport refrigeration system. In mechanically driven transport refrigeration systems the compressor is directly driven by the diesel engine, either through a direct mechanical coupling or a belt drive, and other components, such as the condenser fan(s) and evaporator fan(s) are belt driven. A low voltage unit battery may also be provided to power electronic equipment, such as a system controller and other control system components, as well as lighting associated with the transport refrigeration system. An alternator, belt driven off the diesel engine, is typically provided for charging the low voltage unit battery.
In conventional practice, a transport refrigeration unit installed on a refrigerated truck or trailer operates in one of a temperature pull-down mode, a temperature maintenance mode, or a standstill mode. In the temperature pull-down mode, the refrigerant compressor, the condenser fan(s) and the evaporator fan(s) are operating with the refrigerant compressor generally operating at full capacity to lower the temperature within the cargo space as rapidly as possible to a desired set point temperature appropriate for the particular cargo stowed in the cargo space. In the temperature maintenance mode, the refrigerant compressor, the condenser fan(s) and the evaporator fan(s) are still operating, but the refrigerant compressor is operating at a significantly lower capacity so as to maintain the temperature in the cargo space within a specified range of the desired set point temperature and avoid over cooling. In the temperature maintenance mode, heaters associated with the evaporator may also be activated as necessary to warm the air passed through the evaporators by the evaporator fan(s) to prevent over cooling. In the standstill mode, the refrigerant compressor and the condenser and evaporator fans are off.
Diesel engines used as prime movers on transport refrigeration systems generally have two operating speeds, that is a high RPM speed, such as 2200 RPM, and a low RPM speed, such as 1400 RPM. In operation, the diesel engine is operated at high speed during temperature pull-down and other heavy refrigeration load conditions and at low speed during the temperature maintenance mode. During standstill, the diesel engine is typically idling at low speed. The diesel engine is generally designed to meet the power needs of the transport refrigeration unit during operation at maximum capacity, such as during the temperature pull-down mode, with efficient fuel consumption. Therefore, during the temperature maintenance mode and standstill mode, the diesel engine is operating at lower efficiency and with increased fuel consumption.